The use of foamed plastic material for insulating purposes in building structures, such as exterior or partition walls, bulkheads, ceilings, floors, storage tanks, and roof structures is well known as such foamed plastic materials have a very low thermal conductivity. Such foamed plastic materials having low thermal conductivity may, for instance, comprise phenolic foam material such as, for example, thermoset phenol formaldehyde materials. Other low thermal conductivity foamed materials may comprise polystyrene form, polyurethane foam and/or polyisocyanurate foam. The foamed materials which are used as roofing insulation have a density of from about 1.5 to about 6 lb/ft.sup.3 so as to have sufficient strength to support persons and their equipment.
Typically, in such construction applications the foamed material is sandwiched between a pair of protective skins, such as for example, paper or paper composites. The protective skins serve as a convenient aid in making the heat insulating panels and also protect and maintain the integrity of the foamed material.
One problem experienced with respect to such known heat insulating panels, such as roofing panels, is associated with the curling or warping of the panels. More particularly, during the construction of built-up roofs employing such heat insulating panels, the panels are initially applied, with a suitable adhesive, to the supporting structure, such as for example, to a corrugated steel roof deck, and then bituminized roofing materials are applied over the insulating panels to complete the roof structure.
The curling of the panels appears to be due in part to the fact that foamed material has a tendency to absorb and retain moisture. For example, foam materials have a tendency to absorb approximately 10% moisture by weight when in an environment at 50% relative humidity at room temperature. When such a panel is heated over one surface, for instance by sun radiation, the heat from the sun on this surface can cause a high temperature differential across the surfaces of the panel which can have the effect of driving the moisture in the foamed material away from the hot side toward the cool side of the panel and/or of uneven evaporation of the moisture from the panel. For example, up to a 25.degree. C. differential (60.degree. C. at the upper surface and 35.degree. C. at the lower surface) has been experienced with unprotected heat insulating roof panels. This high temperature differential in part causes the curling or warpage of the panels due to the removal of moisture near the upper surface of the panel which then shrinks relative to the region near the lower surface which has retained a higher amount of moisture. As an example, curling of up to 1 inch has been observed in a 2 inch thick roofing panel constructed of a phenolic foam core sandwiched between a pair of paper sheets, when exposed to full sun on a hot day.
In an attempt to overcome the curling effect in foam panels, it has been proposed to use a thin layer of reflecting material such as aluminum foil on the upper surface of the panel so that the sun radiation tends to be reflected from the upper surface of the panel so reducing the amount of heat which flows through into the foam core. Such a system works well to prevent curling of panels on a roof before they are covered with a roofing membrane; however, after the membrane has been applied, the effectiveness of the reflecting material is reduced and curling of the panel is liable to occur, especially where there is a lack of adhesive between the panel and the roofing structure. Such curling in a built-up roof will wrinkle the membrane.